# ViewData (Website Templates)

ViewData is the **only** allowed input type for Templates in `apps/website`.

Authoritative contract: [`WEBSITE_CONTRACT.md`](docs/architecture/website/WEBSITE_CONTRACT.md:1).

## 1) Definition

ViewData is a JSON-serializable, template-ready data structure:

- primitives (strings/numbers/booleans)
- arrays and plain objects
- `null` for missing values

## 2) What ViewData is NOT

ViewData is not:

- an API Transport DTO (raw transport)
- a ViewModel (client-only class)
- a Display Object instance

## 3) Construction rules

ViewData MUST be created in client code:

1) Initial SSR-safe render: `ViewData = fromDTO(PageDTO)`
2) Post-hydration render: `ViewData = fromViewModel(ViewModel)`

These transformations are Presenters.
See [`PRESENTERS.md`](docs/architecture/website/PRESENTERS.md:1).

Templates MUST NOT compute derived values.

Presenters MUST NOT call the API.

## 4) Determinism rules

Any formatting used to produce ViewData MUST be deterministic.

Forbidden anywhere in formatting code paths:

- `Intl.*`
- `Date.toLocaleString()` / `Date.toLocaleDateString()` / `Date.toLocaleTimeString()`

Reason: SSR and browser outputs can differ.

Localization MUST NOT depend on runtime locale APIs.
If localized strings are required, they MUST be provided as deterministic inputs (for example via API-provided labels or a deterministic code-to-label map) and passed through Presenters into ViewData.

## 5) Relationship to Display Objects

Display Objects are used to implement formatting/mapping, but their instances MUST NOT be stored inside ViewData.

Only primitive outputs produced by Display Objects may appear in ViewData.